If I had to do something like that, which would not be my first choice, I'd probably dress both of us in pink. And then have one of us hold a tiny sign, in pink, that says "It's a boy!"
Just because I like to mess with people sometimes.
Or do everything in a non-gendered color, like green. So it is not obvious what's going on, other than a lot of green, until they focus in on the sign.